1. Field of the Invention
The present invention relates to an overlay accuracy measuring method when patterns are overlaid on a substrate.
2. Description of the Related Art
In the lithographic process for fabricating an semiconductor device and the like, an upper pattern is overlaid on a lower pattern formed on a wafer. This overlay accuracy becomes an important point to improve operating characteristics in the semiconductor device.
Conventionally, according to a method of measuring pattern overlay accuracy, plane images at some points in a lower measuring pattern (such as a box pattern) and an upper measuring pattern (such as a box pattern larger than the lower measuring pattern ) are taken by an optical microscope, and then shifts at a central position (shifts in X and Y directions) are measured for respective measuring patterns, whereby overlay accuracy is measured.
FIGS. 5(a) through 5(c) are views explaining a conventional method. FIG. 5(a) is an enlarged view of a measuring pattern. FIG. 5(b) is a view illustrating measuring points on a wafer. FIG. 5(c) is a view illustrating a measured result.
That is, as shown in FIG. 5(a), a lower box pattern B1 and an upper box pattern B2 are formed as a measuring pattern. Then, shifts Xa, Ya at the central position are measured every measuring point (indicated by .degree.) on a wafer 10 shown in FIG. 5(b).
The plane images of the respective pattern B1, B2 are taken by the optical microscope and the respective central positions are automatically obtained, whereby shifts are calculated. When there are n measuring points, shifts at the n measuring points in X and Y directions (X.sub.1, Y.sub.1, X.sub.2, Y.sub.2, X.sub.3, Y.sub.3 . . . X.sub.n, Y.sub.n) are obtained, and then averages X.sub.ave, Y.sub.ave and standard deviations X.sigma., Y.sigma. are calculated.
However, in this overlay accuracy measuring method, since light and darkness of patterns are measured optically by using the optical microscope, measured values are influenced by image distortion and measurement errors occur.
Among measurement errors, there is an error which absolute values of measured values are not equal when an object is measured by an erected image and an inverted image, namely, TIS (Tool Induced Shift).
As shown in FIG. 3(a), shifts X, Y between the central positions of the patterns B1, B2 are measured by the erected image, and as shown in FIG. 3(b), shifts X', Y' between the central positions of the patterns B1', B2' are measured by the inverted image. In case of .vertline.X.vertline..noteq..vertline.X'.vertline., .vertline.Y.vertline..noteq..vertline.'.vertline., the differences thereof are TISs.
Two correction methods are mentioned to delete those TISs. (1) In one method, TIS is previously obtained every process/step, and the following overlay accuracy is measured while being corrected by the TIS. (2) In another method, the erected image and the inverted image are measured at all measuring points whenever overlay accuracy is measured, and TISs are corrected.
However, according to the one method (1), TIS is constant under the same condition, however, TIS varies since optical characteristics in a measuring apparatus and patterns to be measured vary, therefore, it is impossible to efficiently correct measured values by the TIS previously obtained. Further, according to the another method (2), though the measured value is not influenced by variations of the optical characteristics in the measuring apparatus and of patterns to be measured, there is a problem that it takes a very long for the measuring time.